Magneto-optic recording systems comprise an optical read/write beam arrangement and a magnetizable storage medium, usually a disk. Writing is accomplished by a high intensity focused light beam, such as a laser, which alters the magnetization of the medium by heating a localized area of the medium above its Curie temperature and allowing the area to cool under an applied magnetic field. Reading is accomplished by a lower intensity plane-polarized beam which, upon transmission through and/or reflection from the medium, experiences a rotation in polarization through a characteristic angle .theta. or -.theta. depending on the local magnetization of the medium. Optical detectors may be used to translate the rotation angle into a binary data signal.
Magneto-optic storage media typically consist of a number of thin film layers applied to a substrate. The magnetizable recording layer is generally composed of an amorphous metal alloy having appropriate Curie temperature and coercivity values for good performance in magneto-optic recording.
Many of the elements which are suitable for the amorphous recording layer react strongly with oxygen and other elements which might be present in the immediate vicinity of the media. To protect the media from degradation, a transparent dielectric layer, also known as an interfering or protective layer, is usually deposited on one or both sides of the magnetizable amorphous layer. To be effective, the dielectric materials must not themselves react with the amorphous metal layer or any other layer, must offer chemical and physical resistance to degradation by heat, humidity and corrosive chemicals, and must be transparent at the wavelengths used for reading and writing of data. Such dielectric layers can also provide a thermal barrier to increase recording efficiency, and interference enhancement to increase the magneto-optic rotation angle.
Presently known dielectrics include silicon suboxide (SiO.sub.y, y&lt;2), titanium dioxide, silicon dioxide, cerium oxide, aluminum oxide, aluminum nitride, silicon nitride, silicon carbide, and metal or semi-conductor oxynitrides.
The speed at which the medium can be read and the reliability of the resulting data depend upon the magneto-optic properties of the medium. An important property for optimum performance is the readout carrier to noise ratio (CNR). The CNR is known to be dependent on rotation angle (.theta.) and ellipticity (.epsilon.), as well as the reflectance (R) of the medium.
The thickness of the various layers of the magneto-optic medium, together with the optical properties of the materials used in the medium, affect the performance of the medium. In a conventional magneto-optic medium comprising a magnetic recording layer, two dielectric layers and a reflective layer, R and .epsilon. may be controlled independently by adjusting the thicknesses of the two dielectric layers, while .theta. remains dependent. The media sensitivity, or laser power requirement for reading and writing, can be adjusted independently by varying the thickness of the reflective layer.
A magneto-optic drive is generally designed to operate best when these media properties are within certain ranges. Therefore, the construction of a magneto-optic medium is often "tuned" for satisfactory performance at the specific laser beam wavelength .lambda. of a particular drive. Current drives generally contain lasers having wavelengths in the range of approximately 780 nm to 830 nm. The parameters R, .theta., and .epsilon. vary significantly with .lambda., meaning that a medium suited for use at long laser beam wavelengths may not be compatible with a drive having a shorter wavelength laser.